There is known a receiver which combines several components of multi-path signals that are mutually delayed by different time delays before reaching the receiver. Such a receiver is for example present in code division multiple access (CDMA) wireless communication systems and is currently designated by the name of Rake-type receiver or “Rake” receiver.
In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the conventional multiple access systems for delivering simultaneous services to a certain number of terminals. The basic idea underlying the FDMA and TDMA systems is dividing the available resource into several frequencies or into several time slots, respectively, such that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception take place at different frequencies and also in different time slots.
Unlike these systems using frequency division or time division, CDMA systems enable multiple users to share a common frequency and a common time channel by using a coded modulation. Amongst the CDMA systems are the CDMA 2000 system, the WCDMA system (wide band CDMA), and the IS-95 standard.
In CDMA systems, a scrambling code is associated with each base station and is used to distinguish one base station from another. In addition, an orthogonal code, known as the OVSF code, is allocated to each remote terminal (such as a cellular mobile telephone). All the OVSF codes are mutually orthogonal which distinguishes one channel from another.
Before transmitting a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the channel.
In CDMA systems, those that use a distinct frequency for transmission and reception (CDMA-FDD system) can be distinguished from those which use a common frequency for transmission and reception, but distinct time domains for transmission and reception (CDMA-TDD system).
The present invention applies advantageously to communication systems of the CDMA type, and is particularly suited to systems of the WCDMA type with terrestrial radio access (UTRA FDD/TDD).
The incident signal received by a mobile telephone for example comprises different versions delayed in time from the signal initially transmitted, versions or echoes which are the result of the multi-path transmission characteristics of the transmission environment between a base station and the telephone, with each path introducing a different delay.
The “Rake” receiver in a cellular mobile telephone operating in a CDMA communication system is used to carry out temporal alignment, descrambling, compression, channel correction and combination of the delayed versions of the initial signals in order to deliver the information streams (symbols) contained in the initial signals.
A “Rake” receiver comprises several fingers, with each finger being intended to demodulate a given path received at a given instant.
Furthermore, the receiver comprises a channel estimation unit the purpose of which is to identify the various echoes through their delay and their mean energy as well as a mechanism for selecting echoes with a view to their respective assignment to the fingers of the Rake receiver.
Generally, the number of echoes detected is greater than the processing capacity of the receiver (number of fingers) for hardware complexity limitation reasons.
Hence, at the present time use is made of a selection mechanism capable of selecting from among the echoes detected the N best (N being the number of fingers of the receiver).
Generally, the selection of the echoes is based on the power profile measured by the receiver. More precisely, the receiver estimates the impulse response of the channel, based on the pilot signal transmitted by the base station. From this impulse response, it forms its energy, referred to as the “power profile”, and lists this estimation in a non-coherent manner over a given measurement duration.
The echoes which constitute the radio propagation profile between the base station and the mobile receiver are subsequently estimated with the help of this power profile.
Generally, the fingers are associated with the N maxima of the power profile, while rejecting any other maximum which might be close to a maximum retained by a duration of less than the duration of a chip.
It is recalled in this regard that in CDMA systems in particular, the symbols are transmitted within successive frames, with each frame being subdivided into a certain number of time slots. Each time slot conveys a certain number of symbols, with each symbol consisting of a predetermined number of chips.
This method of selecting echoes operates well in the case where the radio propagation profile exhibits echoes actually separated by more than a duration of a chip. However, in the case where close echoes are present, there is an irremediable loss due to the exclusion of such echoes.
As a consequence, to increase the performance of the receiver, it is necessary to search for the maxima in the power profile without applying the rejection principle described above.
However, there is the problem of selecting false echoes, that is to say the echoes which, on account of oversampling, are nothing other than replicas of the shaping filter and therefore afford no additional information.
If a false echo such as this is actually selected and assigned to a finger of the receiver, it then gives rise to needless energy consumption and worse still, due to the limitation of the hardware complexity of the receiver, the exclusion of the use of other true echoes that are less powerful than the false echo selected, but which afford more information.
This ultimately gives rise to a lowering of the performance of the receiver.